The present invention is directed to quantum information technology, e.g., quantum cryptography. It relates to an efficient, room-temperature source of polarized single photons and more particularly to such a source using deterministically aligned single emitters in a planar aligned liquid crystal host.
Quantum information in the form of quantum communications and quantum computing is an exceedingly active field today. See, for instance, the following books: M. A. Nielsen and I. L. Chuang, Quantum computation and quantum information, Cambridge: Cambridge Univ. Press, 2001, D. Bouwmeester, A. Ekert, A. Zeilinger, Eds., The physics of quantum information: quantum cryptography, quantum teleportation, quantum computation, Springer: Berlin, 2000, and the following review paper: N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Rev. Mod. Phys., vol. 74, 145. 2002. Numerous theoretical concepts promise powerful quantum-mechanics-based tools that, to date, wait for realization pending the arrival of reliable hardware. A single-photon source (SPS) that efficiently produces photons with antibunching characteristics is one such pivotal hardware element for quantum information technology. Using a SPS, secure quantum communication will prevent any potential eavesdropper from intercepting a message without the receiver's noticing. (E. Klarreich, Nature, vol. 418, 270-272, 2002)
In another implementation, a SPS becomes the key hardware element for quantum computers with linear optical elements and photodetectors (See the following paper: E. Knill, R. Laflamme, and G. J. Milburn, Nature, vol. 409, 46-52, 2001). Again, its practical realization is held back in part by difficulties in developing robust sources of antibunched photons on demand.
In spite of several solutions for SPSs presented in the literature, significant drawbacks remain. They are the reason for current quantum communication systems being baud-rate bottlenecked so that photon numbers from ordinary photon sources may be attenuated to the single-photon level (˜0.1 photon per pulse on average). An efficient (with an order of magnitude higher photon number per pulse) and reliable light source that delivers a train of pulses containing one and one photon only is a very timely challenge. To meet this challenge, several issues need addressing, from achieving full control of the quantum properties of the source to easy handling and integrability of these properties in a practical quantum computer and/or communication setup. In addition, in quantum information systems it is desirable to deal with single photons synchronized to an external clock, namely, triggerable single photons. Polarization states of single photons are also important as they enable polarization-qubit encoding of information.
The critical issue in producing single photons by a method other than by trivial attenuation of a beam is the very low concentration of photons emitters dispersed in a host, such that within a laser focal spot only one emitter becomes excited which can emit only one photon at a time. Most current SPSs, e.g., based on semiconductor heterostructures, operate only at liquid He temperature—a major impediment to widespread use. (See the following papers: J. Kim, O. Benson, H. Kan, Y. A. Yamamoto, Nature, vol. 397, 500-503, 1999; A. Imamoglu and Y. Yamamoto, Phys. Rev. Lett., vol. 72, 210-213, 1994; E. Moreau, I. Robert, J. M. Gérard, I. Abram, L. Manin, V. Thiery-Mieg, Appl. Phys. Lett., vol. 79, 2865-2867, 2001; P. Michler, A. Kiraz, C. Becher, W. V. Schoenfeld, P. M. Petroff, L. Zhang, E. Hu, A. Imamoglu, Science, vol. 290, 2282-2285, 2000; C. Santori, M. Pelton, G. Solomon, Y. Dale, and Y. Yamamoto, Phys. Rev. Lett., vol. 86, 1502-1505, 2001; M. Pelton, C. Santori, J. Vu{hacek over (c)}ković, B. Zhang, G. S. Solomon, J. Plant, and Y. Yamamoto, Phys. Rev. Lett., vol. 89, 233602, 2002; Z. L. Yuan, B. E. Kardynal, R. M. Stevenson, A. J. Shields, C. J. Lobo, K. Cooper, N. S. Beattie, D. A. Ritchie, M. Pepper, Science, vol. 295, 102-105, 2002).
Of the known room-temperature (RT) SPSs, only those based on single-dye-molecule fluorescence can be used in much higher speed systems than other RTSPSs. This SPS was developed in the following papers: W. P. Ambrose, P. M. Goodwin, J. Enderlein, D. J. Semin, J. C. Martin, R. A. Keller, Chem. Phys. Lett., vol. 269, 365-370, 1997; L. Fleury, J.-M. Segura, G. Zumofen, B. Hecht, and U. P. Wild, Phys. Rev. Lett., vol. 84, 1148-1151, 2000; B. Lounis and W. E. Moerner, Nature, vol. 407, 491-493, 2000; F. Treussart, A. Clouqueur, C. Grossman, and J.-F. Roch, Opt. Lett., vol. 26, 1504-1506, 2001; F. Treussart, R. Alleaume, V. Le Floch, L. T. Xiao, J. M. Courty, J. F. Roch, Phys. Rev. Lett., vol. 89, no. 9, 093601-4, 2002, and in the US Patent Application Publication No. 2002/0146052 A1, Oct. 10, 2002 by W. E. Moerner and B. Lounis. Alternatives such as color centers in diamond and colloidal semiconductor CdSe—ZnS quantum dots possess unacceptably long fluorescence lifetimes. For instance, the diamond color center has a 11.6-ns and 22.7 ns fluorescence life time in mono- and polycrystal, and CdSe—ZnS quantum dots one of ˜22 ns. (See, for example, the following papers: C. Kurtsiefer, S. Mayer, P. Zarda and H. Weinfurter, “Phys. Rev. Lett., vol. 85, 290-293, 2000; R. Brouri, A. Beveratos, J.-P. Poizat and P. Grangier, Opt. Lett., vol. 25, 1294-1296, 2000; A. Beveratos, R. Brouri, T. Gacoin, J.-P. Poizat, and P. Grangier, Phys. Rev. A, vol. 64, 061802(R), 2001; A. Beveratos, S. Kuhn, R. Brouri, T. Gacoin, J. P. Poizat, P. Grangier, Europ. Phys. Journ. D, vol. 18, 191-196, 2002; A. Beveratos, R. Brouri, T. Gacoin, A. Villing, J. P. Poizat, P. Granger, Phys. Rev. Lett., vol. 89, no. 18, 187901-4, 2002; P. Michler, A. Imamoglu, M. D. Mason, P. J. Carson, G. F. Strouse, and S. K. Buratto, Nature, vol. 406, 968-970, 2000; B. Lounis, H. A. Bechtel, D. Gerion, P. Alivisatos, W. E. Moemer, Chem. Phys. Lett., vol. 329, 399-404, 2000; G. Messin, J. P. Hermier, E. Giacobino, P. Desbiolles, and M. Dahan, Opt. Lett., vol. 26, 1891-1893, 2001).
The key advantage of dye molecules is that their excited-state life-time of only a few nanoseconds permits excitation repetition rates above ˜100 MHz. In dye-based SPSs, one of the current challenges is dye bleaching. However, recently single terrylene molecules have been doped into p-terphenyl molecular crystals (10−11 moles of terrylene per mole of p-terphenyl) prepared by a sublimation procedure that produced tiny platelets. In this host, the dye is protected from exposure to diffusing quenchers (such as oxygen), and benefits from strong phonon emission into the host, preventing rapid thermal decomposition of the dye under intense illumination (B. Lounis and W. E. Moerner, Nature, vol. 407, 491-493, 2000; the US Patent Application 20020146052 A1, Oct. 10, 2002 by W. E. Moemer and B. Lounis; L. Fleury, J.-M. Segura, G. Zumofen, B. Hecht, and U. P. Wild, Phys. Rev. Lett., vol. 84, 1148-1151, 2000). For “thick” p-terphenyl crystals (˜10 μm), this system becomes extremely photostable, allowing hours of continuous illumination of individual molecules without photobleaching. It assures long-term spectral stability and reproducibility from one terrylene absorber to the next. Pumped by periodic, short-pulse laser radiation, single photons were generated at predetermined times at pump-pulse-repetition rates within the accuracy of the emission lifetime (˜3.8 ns). Technical implementation of this system is difficult as these monoclinic, sublimation-produced microcrystals are stress sensitive and fragile. In addition, terrylene's molecular dipole moment in the p-terphenyl host crystal takes on an orientation perpendicular to the platelet's surface (i.e., perpendicular to the incident light's E-field). This, in turn, leads to poor coupling with the polarized excitation light, prompting poor fluorescence emission even at high excitation intensities (saturation intensity is about 1 MW/cm2 at room temperature).
In spite of the elegance of the terrylene/p-terphenyl experiments, this technology must be considered unrealistic for practical application. Its weak point is also a background from “ordinary photons” from out-of-focus molecules or Raman scattering, because of the very high pumping intensities required. Emitted photons are not polarized deterministically (there is no known, efficient method for aligning rapidly a multitude of micrometer-sized, monoclinic crystallites relative to one another). Note that noncrystalline, amorphous hosts, e.g., polymers, do not (1) offer the same spectral stability in single-molecule emission even in the case of terrylene, (2) provide long-time protection against bleaching. To date, no crystal hosts other than the fragile, sublimated p-terphenyl flakes have been proposed in single-dye-molecule room-temperature experiments.